The present invention generally relates to electrical machines, such as high speed aerospace generators and motors, and, more particularly, to mounting assemblies that interface between the bearing assemblies of the rotor shaft and the support housing of the electrical machines.
Aircraft systems include various types of rotating electrical machines, such as alternating current (AC) motors and generators of various designs. Generally, the electrical machine includes a rotor and a stator. The rotor is part of a rotating shaft assembly supported by bearings. The stator is part of a static assembly that supports the rotor. For current electrical machines, the bearings are directly mounted to the static structure, often referred to as the housing, or have a hydraulic damper between the bearing outer race and the housing mounting bore (see U.S. Pat. No. 6,747,383).
If the machine is operated as a motor, electrical power is supplied to the stator to develop a rotating electrical field. This rotating electrical field generates a torque in the rotor causing it to rotate. If the machine is operated as a generator, electrical power is supplied to the rotor to generate a magnetic field. The generated magnetic field rotates as the rotor rotates. This rotating magnetic field induces a voltage across the stator, which supplies electrical power to a load.
The future direction of aerospace quality electric power systems is towards higher power, higher speed, lighter weight, variable frequency electric generators and starter generators. Variable frequency generators rotate throughout a range of speeds within an operating speed range. For high speed aerospace generators, the operating speed range is typically 7,200 to 30,000 rpm. Potentially large centrifugal forces can be imposed on the rotors of generators operating at such speeds. The generator rotors must be precisely balanced to avoid vibration, which may lead to deviation of the rotor shaft axis from its intended axis of rotation. Practically achieving and maintaining this precision balance can be difficult due to variations in the manufacture and assembly process. The amplitudes of vibrations resulting from rotor out of balance can be significant if the rotor's rotational speed reaches its resonance speed, or a multiple of its resonant speed. Such speeds are generally referred to as “critical speeds”. Rotor critical speed and machine response is a function of several variables including the rotor mass, the distribution of that mass, the flexibility of the shaft, the bearing support locations and the stiffness of the rotor, bearings, housing and interface.
Typical aerospace generators and starter generators employ rolling element bearings, which have very high stiffness and little damping. If an unbalanced rotor is rotating for prolonged periods of time at one of its critical speeds, it may be damaged, even catastrophically. If one or more of the rotor critical speeds are below the operating range, to avoid damage, the rotor may be quickly brought through a critical speed into the operating speed range. If the critical speed is well damped, the rotor may pass through the critical speed slowly without experiencing high excursions or bearing loads.
In U.S. Pat. No. 4,553,855, a support assembly for a rotating shaft is disclosed. The support assembly comprises a spring and a squeeze film damper (SFD). The support assembly uses a series of support rods spaced on the interior and exterior sides of an annular spring to define spring segments that act as the spring for supporting the journal. The SFD comprises a cavity incorporated either as part of the spring support structure or separately within the assembly. Although the described assemblies may be used to provide the necessary damping, the positioning of the plurality of support rods increases installation time.
In U.S. Pat. No. 5,603,574, a fluid-damped support for a bearing is disclosed. The fluid-damped support comprises a spring and a squeeze film damper (SFD). The described support utilizes a unique combination of springs and dampers as an integral part of the bearing support structure. Radial structural members form the springs and cavities filled with oil form the dampers. Although the described support may limit or damp out the vibrations occurring when the supported member passes through natural frequencies before reaching operating speed, the '574 patent design is complex, requires radial and axial space for implementation and can only be tuned during design for a particular application.
As can be seen, there is a need for a support assembly for an electrical machine rotor that can move the rotor critical speeds outside of the extremes of the operating speed range and that can damp out the rotor responses as the rotor passes through these critical speeds. Further, there is a need for a rotor mounting system that has a simple design and that is easy to manufacture and install.